Electrical installations include electrical power sources and electrical loads, such as electric motors. Such electric motors are, for instance, medium voltage or low voltage electric motors, which typically means motors operating up to around 20 kV. Loads other than electrical motors can also be connected to the power sources.
Such electrical installations are generally provided with control circuitry. Such control circuitry generally includes a protection device for disconnecting the electric motors from the power source based on the current drawn by the electric motor. Generally, the protection device is configured to interrupt the current feeding the electric motor when the value of the current exceeds a predetermined threshold.
During the startup of the electric motors, and before reaching steady-state, the electrical current drawn by such electric motors is generally higher than their nominal current. Therefore, during the startup of the electric motor, the threshold is generally set to several times the nominal current to prevent the protection device from disconnecting the electric motor. Then, the current drawn by the electric motor decreases over time until it reaches the nominal current of the electric motor when the electric motor reaches steady-state. For instance, the predetermined threshold is then lowered to a value that is slightly higher than the nominal current of the electric motor. This method is known as “cold load pick-up”.
When several electric motors are provided in the electrical circuit, for instance when several electric motors are connected through a power bus (also called feeder) to a same electric power supply or power source, the electric motors are generally started in a sequence in order to avoid voltage dips that may damage the power supply. Electric motors can also be started in a sequence due to industrial process purposes.
In such cases, the threshold is generally set to several times the sum of the nominal currents of the electric motors. Then, the threshold is lowered when all the electric motors are assumed to be in steady-state. Alternatively, the threshold is generally raised during the startup of each electric motor of the sequence, following a staircase pattern.
The documents U.S. Pat. Nos. 3,129,358 and 8,279,565 disclose such methods.
However, with such methods, if a fault occurs during the startup sequence, the fault is generally not detected until the end of the startup sequence, as the fault current is generally smaller than the threshold.
Therefore, these methods entail a risk of blindness if a fault occurs during the startup sequence, that is to say that the fault is generally not detected before the end of the startup sequence. Therefore, these methods lead to a lack of reactivity of the protection device.
The purpose of the invention is to propose a method for operating a protection device that allows for a better detection of faults, especially during the startup of cold loads such as electric motors.